This invention relates to nucleated propylene polymer compositions.
It is readily apparent that many applications for plastic materials used as films and coatings require substantial clarity. Clarity, however, is also an asset in other applications including injection molding of relative thick parts.
Polypropylene is becoming a major plastic for use in injection molding. Polypropylene, however, tends to be opaque in the absence of a clarifying or nucleating agent. Apparently nucleating agents work by bringing about the formation of a large number of small crystals instead of the formation of large spherulite crystals. Nucleation of polypropylene is not a simple matter, however. Solid nucleating agents such as sodium benzoate which apparently work by forming small discrete particles around which crystals form, tend to be limited in the concentration to which they can be used without being counter-productive. In addition, polypropylene must be stabilized and some combinations of stabilizers, nucleating agents and catalyst residue can actually cause degradation or at least reduce the clarifying effectiveness of the nucleating agent.
One particularly desirable class of clarifying agents known in the art is that represented by alditols and substituted alditols such as dibenzylidene sorbitol, which is sold by Milliken Chemicals under the tradename Millad 3905. These materials can be melted at polymer processing temperatures and can be more uniformly distributed than solid discrete particles and thus can be used in sufficiently high concentration to provide a more effective clarifying or nucleating effect. However, the presence of catalyst residues has an adverse interaction with such materials thus limiting their usefulness as a practical matter.